The present invention relates to a thermal processing method and an apparatus therefor.
During the fabrication of a semiconductor wafer, the wafer undergoes various processes such as oxide diffusion and chemical vapor deposition (CVD). A particular problem is caused by the way in which design rules of semiconductor devices are continuing to become finer (from 0.4 .mu.m to 0.2 .mu.m) whilst the semiconductor wafers are becoming even larger in size (from eight inches to 12 inches). For these reasons, there is intense interest in developing a rapid thermal processing (heat treatment) apparatus that can be used with the ultra-thin film creation technology used for wafers of such large areas.
An essential condition during the processing of a semiconductor wafer is to reduce the thermal budget. During 50 to 100 .ANG. doping or the creation of an ultra-fine gate oxide film or capacitor isolation film, rapid heating (in other words, heating within a short period of time) is essential during 50 to 100 .ANG. doping or the creation of an ultra-fine gate oxide film or capacitor isolation film, for example.
In order to design lower resistances on thinner PN junctions of less than 0.1 .mu.m and enable junction fabrication on any desired shape of surface, for instance, it is necessary to prevent the films from deteriorating and crystal defects from generating when the junction is made. However, the active region of the PN junction is so narrow, rapid thermal processing is necessary.
In addition, during the creation of a film such as a LOCOS oxide film, compression stresses in the neighboring LOCOS oxide film are increased by a multiplication effect due to thermal cycling, so that the reliability of characteristics such as changes in surface potential, leakage current, and withstand voltage is liable to drop. In order to prevent this, it is necessary to reduce thermal cycling by using rapid thermal processing.
When forming capacitor isolation films using a highly dielectric material, it has become essential to use systems that provide compound processing which includes doping and metal film creation processes that enable the formation of films of substances such as metal oxides (such as Ta.sub.2 O.sub.3) and polyamides (passivation films).
Since the diameters of semiconductor wafers are currently increasing from eight inches to 12 inches, it is necessary to facilitate a uniform, rapid thermal processing that minimizes temperature differences between the center and periphery of a semiconductor wafer, create designs that reduce the slippage, distortion, and warping that are liable to occur in semiconductor operations, and also ensure that defects do not occur during the fabrication of semiconductor devices.
However, in a conventional longitudinal batch type of thermal processing apparatus, a cylindrical heat source is positioned around a stack of semiconductor wafers held in a wafer boat made of high purity quartz, and heat from this source is directed from the peripheral parts of the wafers toward the centers thereof. This causes a problem in that uniform thermal processing is impossible because any attempt made to heat the semiconductor wafers rapidly will generate a large thermal gradient between the center and periphery of each semiconductor wafer.
These circumstances led the inventors of the present invention to continue their research into technology designed to ensure a uniform temperature within a semiconductor wafer, by positioning a heat source at the other end of a cylindrical process tube which has at its one end an entrance/exit, and thermally processing a workpiece at a prescribed position to which it is brought from the entrance/exit of the process tube. They have ascertained that, even with this thermal processing method, simply bringing the semiconductor wafer as it is from the entrance/exit of the process tube is not sufficient to bring the temperature of the semiconductor wafer to the prescribed processing temperature, and time is required until the temperature stabilizes at the prescribed processing temperature. They have also identified another problem in that, if the actual temperature of the workpiece placed at the prescribed position should deviate from the prescribed processing temperature while the workpiece is being processed, temperature control by the heat source makes it difficult to quickly return the temperature to the prescribed processing temperature.